De Maio

COVID-19 pandemic has shown a significant disruptive impact on Tuberculosis (TB) services, with negative effects on prompt diagnosis, treatment and immunization.[1,2] Pressure on laboratories and pharmaceutical industries led to the readaptation of many TB labs to detect SARS-CoV-2 as well as Bacillus Calmette-Guérin (BCG) shortages and consequent decrease of newborn vaccinations.[3] Estimates indicate a 25% drop in the global BCG coverage and an increase in pediatric deaths ranging from 0.5% to 17%.[4]
In several countries, reports suggest a decline in case notification in the last few months due to massive cancellation of routine health services in many
settings.[5-7] Although it has been noted that many of the preventive measures implemented to reduce SARS-CoV-2 incidence also have a clear benefit on reducing Mycobacterium tuberculosis (Mtb) transmission, 2020 saw the first year-over-year increase in TB deaths from 2005, regardless of physical distancing and PPE (personal protective equipment) wearing measures.[2,8]
In Canada, the pandemic significantly affected latent TB infection (LTBI) and active TB treatment, leading to ineffective measures for TB elimination.[9] In Spain, newly diagnosed TB patients had more extended pulmonary disease, moreover there was a rise in household transmission probably due to anti-COVID-19 measures.[10] Also, in England, it has been observed a fall in rates of TB treatment initiation during the period of government-imposed lockdown (March 23–May 10, 2020), and an increase of cases of disseminated TB during the COVID-19 pandemic.[11] All this makes it important to evaluate the measures against COVID-19 globally and not only considering the pathologies related to SARS-CoV-2,
The COVID-19 emergence prompted the scientific community to focus on determining the mechanisms of transmission, the identification of virulence factors of SARS-CoV-2 and the development of suitable therapies.[12] Therapeutical management of COVID-19 is in constant change, and treatment guidelines are readily updated based on scientific evidence and experts’ opinion (National Institutes of Health, n.d.) as we entered in an era of “hype-based medicine”,[13] the long forgotten eminence-based medicine regained importance as the number of trials on possible therapies multiplied, some of them causing overnight changes in the management of COVID-19 patients.[14] The lack of antiviral therapies and the rapid spread of the infection convinced investigators and pharmaceutical companies to focus on the development of vaccines,[15] able to induce neutralizing antibodies against SARS-CoV-2 Spike protein in naive subjects. The developed vaccines do not only trigger a humoral response against the protein, but they impact all the components of the immune response.
As most of the therapies used against COVID-19 disease therapies do not target SARS-CoV-2, but aim to regulate the host immune response[16,17] it is reasonable to consider the long-term effects of these therapies on subjects with latent TB infection (LTBI).
In this commentary, we aim to summarize treatments against SARS-CoV-2 and underline their possible effects on Mtb infection highlighting likely “side” effects that could help to contain virus-mediated damage and, conversely, prompt mycobacterial replication in both early infection or during Mtb latency.

SARS-CoV-2 represents the biggest therapeutic challenge of our century. At present, approximately 2900 clinical trials have been registered,[14] designing new molecules and repurposing existing drugs based on the virus biology and pathogenesis.
Therapeutical approaches range from convalescent plasma of people who have recovered from COVID-19, to medications which are commonly used to treat autoimmune or inflammatory diseases as well as drugs used to treat other infections.[18,19]
Pharmaceuticals used for COVID‑19 target different pathogenetic mechanisms, with the aim of a) blocking viral replication, summarized in points 1-3 of the Figure 1, and b) reducing tissue damage, modulating the immune responses, and preventing over-inflammation (Figure 1, points 4-6).

Figure 1. Schematic representation of the pharmaceuticals used against SARS-CoV-2 infection. The first class of molecules includes antivirals to prevent viral entry (point 1); the second class includes compounds that inhibit gene transcription (point 2) and the third class accounts molecules that prevent proteolytic processing and block viral docking (point 3). The points 4-6 described medications that reduce tissue damage, modulating the immune responses or preventing over-inflammation.

The first class includes antivirals to prevent spike-protein-mediated cell fusion, thus blocking viral entry (Figure 1, point 1), inhibit gene transcription (Figure 1, point 2) or prevent proteolytic processing and block viral docking (Figure 1, point 3), as explained in Table 1.[20-23] Interestingly, several agents show no effects on SARS-CoV-2 even though they were described to have activity against other infections.[24]

Table 1. Summary reporting experimental evidence of the impact of drugs used against SARS-CoV-2 infection on Mycobacterium tuberculosis infection.

SARS-CoV-2 infection causes an overproduction of type I interferons triggering the transcription of several genes and the recruitment of CD4+ T helper lymphocytes, responsible for the Th1/Th2 response.[25] For this reason, immunomodulators (corticosteroids, interferons, monoclonal antibodies against inflammatory cytokines) have been suggested, and largely used, to reduce the over-inflammation that is responsible for several systemic disease manifestations.
However, the NHS Panel failed to evaluate the real role of some of these therapies due to insufficient evidence to recommend either for or against their use.[26]
Examples of drugs in this category are IL-1 inhibitors, colchicine, the antiparasitic agent ivermectin,[12] and thalidomide.[27] Some others are currently recommended as IL-6 inhibitors and Janus Kinase inhibitors (refer to Table 1 and Figure 1, point 4 to 6).
Although, the use of immunomodulatory treatments had an immediate impact on the care of patients infected with SARS-CoV-2, their long-term effects are unknown.

Mtb infection represents a classical model of persistent infection, a situation in which a microorganism can persist indefinitely within the host,[28,29] establishing an equilibrium between the pathogen and the host immune response whose modification could increase the risk of relapse and disease. Indeed, host immune response can limit Mtb spread, after macrophages killing evasion, creating a multicellular structure known as granuloma,[30] which entraps mycobacteria that persist in a heterogeneous range of states.[31] In the last decades, to deal with the emergence of Mtb strains resistant anti-TB drugs (MDR/RR-Mtb and XDR-Mtb), a novel approach has been proposed targeting the host and so named host directed therapies (HDTs).[32–35] HDTs can support antimycobacterial host response at different stages: a) perturbating granuloma integrity to enhance drug penetration; b) modifying autophagy or phagosome maturation to increase intracellular killing; c) promoting cell-mediated response; d) inducing antimicrobial peptides and controlling inflammation response by avoiding tissue damage.[36] While the use of HDTs seem to support anti-TB treatment in symptomatic individuals,[37] no data nor anecdotal knowledge support the use of such therapies in people with asymptomatic or subclinical infection.[38]
In other words, it is undeniable that some immunomodulatory treatments may alter the host-Mtb equilibrium, favoring either the host or the bacteria. In this scenario, we cannot exclude that those immunomodulatory therapies used against COVID-19 may have a negative effect on infected individuals causing symptomatic TB.
A recent paper highlighted the relationship between SARS-CoV-2 and Mtb infection, showing that asymptomatic SARS-CoV-2 seropositive individuals with a positive IGRA exhibited heightened levels of humoral, cytokine production, and systemic inflammation compared to individuals negative for Mtb infection.[39] Mtb is apparently able to modulate the host immune response in SARS-CoV-2-infected individuals. Furthermore, various clinical cases describe TB reactivation following SARS-CoV-2 infection confirming the concerns that COVID-19 associated CD4+ T-cell depletion or altered T-cell function can have similar implications as HIV for TB disease progression, promoting the development of active TB.[25,40] Moreover, some studies highlighted a higher probability to develop severe disease in patients with SARS-CoV-2 / Mtb co-infection compared to COVID-19 patients.[41,42] Unfortunately, we have little information on TB occurrence after COVID-19 treatments.[43] On the other hand, there was a delay in the onset of the pandemics in many countries endemic for TB. Moreover, those countries showed lower COVID-19’s severe cases and SARS-CoV-2 related-mortality.[44] Intriguingly, one of the variables that was mathematically linked to COVID-19 low spread was BCG vaccination,[45] which is known to stimulate non-specific heterologous immune responses inducing cross-protective effects toward non-tuberculosis-related diseases, included SARS-CoV-2.[15,46,47] Indeed, numerous clinical trials are currently registered to update on the benefits of BCG vaccinations against SARS-CoV-2 exposure.[15,47]
We can classify therapies used against COVID-19 based on their activity on Mtb infection in four main drug classes: a) drugs acting directly on Mtb (Figure 2, point 1); b) drugs that modify phagosome acidification (Figure 2, point 2); c) drugs with adjuvant function that can indirectly modulate the infection (Figure 2, point 3) and d) drugs with immunomodulatory activity (Figure 2, point 4). Although many anti-COVID-19 pharmaceutics appeared to impair mycobacterial growth in in vitro experiments, (Figure 2, point 1),[48–51] we focus our attention on their immunomodulatory effects (Table 1).

Figure 2. Schematic representation of the medications used against SARS-CoV-2 with effects on Mycobacterium tuberculosis infection. Therapies used against COVID-19 are classified based on their activity on Mtb infection in four main classes: drugs acting directly on mycobacteria (point 1) or indirectly showing ability to modify phagosome acidification (point 2), to modulate the infection with adjuvant functions (point 3) and to regulate hose immune response (point 4).

Hydroxychloroquine inactivates mycobacterial NAD+ dependent DNA ligase A[52] and modulates phagolysosome, reducing intracellular mycobacterial growth.[48] Similarly, chlorpromazine, an antipsychotic drug, has been observed to have antimycobacterial activity and to promote macrophagic killing by increasing phagosome acidification.[53] Acidification may though differently affect phagosomes at different stages of maturation.[54] Moreover, Mtb itself can influence phagosome maturation potentially counteracting these drugs.[55]
Nitazoxanide, which has been also suggested to modulate immune response inducing interferon-γ,[50,56] inhibits intracellular Mtb growth while amplifying Mtb-induced gene expression.
Thalidomide represents a compound that has been tested against Mtb infection showing a detrimental effect on infection control due to TNF-α inhibition with consequent increase in mycobacterial replication[57,58] (Figure 2 and Table 1).
Interestingly, several drugs that have been proposed against SARS-CoV-2 have not been tested in vitro against Mtb infection (Table 1). This is true for several molecules that act on the host immune system to prevent over inflammation (which has been observed as a critical point for the progression of SARS-CoV-2 infection). These compounds, that dampen pro-inflammatory cytokines, could impair the fine equilibrium between Mtb replication and the host immune system response, thus promoting active disease. Among them, monoclonal antibodies such as IL-6 antagonists and antivirals have been observed to significantly modulate host cytokine response and potentially alter host immune response versus Mtb replication.[59] Interestingly, Mtb regulates IL-6 secretion to inhibit type I interferon signaling and causes disease progression which appears to be associated to sigH gene expression.[60] For this reason, IL-6 antagonist could have important implications during Mtb infection.
Another example are corticosteroids that are beneficial in hospitalized COVID-19 patients,[61,62] but, conversely, could increase the risk of LTBI reactivation or progression of sub-clinical TB.

The Dark Side of the COVID-19 Treatments on Mycobacterium tuberculosis Infection